The invention relates to the field of photolithography, and more particularly to a system for applying modifications to an IC layout.
Photomasks are used in the production of integrated circuits (ICs) to transfer a circuit layout (xe2x80x9cIC layoutxe2x80x9d) onto a receiving substrate, such as a silicon wafer. A photomask is typically a glass plate covered by a thin chrome layer, in which a portion of the IC layout is etched. A source of light or radiation is used to expose this photomask pattern (xe2x80x9cphotomask layoutxe2x80x9d) onto, a layer of photosensitive resist on the surface of the wafer. The top surface of the wafer is then chemically etched away in the areas not covered by the photoresist, thereby completing the transfer of the original IC layout onto the wafer. This transference process is known as photolithography.
Photolithography is a critical step in the IC manufacturing process. The accuracy of the pattern formed on the wafer (xe2x80x9cprinted imagexe2x80x9d) significantly affects both process yield and IC performance. However, reliably resolving modern IC geometries in the 0.25-xcexcm range and smaller using existing photolithography equipment is difficult. At such reduced feature sizes, optical distortion impedes exact transfer of the photomask pattern onto the wafer.
To counteract any unwanted deviations, a technique known as optical proximity correction (OPC) has been developed. OPC involves modification of the original IC layout to compensate for distortions introduced by the exposure process. The modification involves changing the dimensions of affected features or incorporating corrective features into the photomask pattern. For example, diffractive effects around small features in a photomask pattern can cause corner rounding in the printed image. FIG. 1a shows a sharp corner 100 that would typically be present in an original photomask pattern, and a rounded corner 110 that would appear in the actual printed image without OPC. FIG. 1b shows a corner 120 in a photomask pattern that has been modified using OPC to include an extra feature called a serif. The serif is sized such that the printed image corner 130 produced by photomask corner 120 has the desired form. Other OPC features (hammerheads, scattering bars, assist features, etc.) are well known in the art. These modifications to the original IC layout may sometimes be referred to generically as xe2x80x9cbiasesxe2x80x9d or xe2x80x9cbias featuresxe2x80x9d.
There are two methods for determining the appropriate biases to add to an IC layout: model-based OPC and rule-based OPC. Model-based OPC applies corrections to the IC layout and uses models of the photolithography processes to determine the most effective biases. By essentially simulating an actual photolithography step and converging on a solution, model-based OPC can provide a thorough and detailed set of biases to apply to the original layout. However, this technique is extremely computation-intensive due to the iterative nature of the process. Also, the accuracy of the final output is only as good as the models used in the calculations.
In contrast, rule-based OPC applies a set of geometry-based rules to the original IC layout. The rules are specified by the user, and define the biases to be incorporated in the photomask layout. Conventional rule-based OPC systems base the application of OPC biases on the geometries of edges within the original IC layout.
Although the complex patterns of an IC layout may seem to be made up of fine lines, even the thinnest of lines are actually 2-dimensional elements. As such, they can be represented by a series of contiguous edges, joined to other edges at distinct vertices. Conventional rule-based OPC systems evaluate each edge of the original IC layout and apply biases to those edges according to pre-specified rules. The rules are typically based upon the length of an edge and its spacing from other edges.
FIG. 2a shows a layout feature 200a, sometimes referred to as a xe2x80x9cfingerxe2x80x9d, which is common in IC layouts. OPC modifications must often be applied to finger features to create photomask layouts that properly transfer the original feature during photolithography. Feature 200a comprises edges 221, 222, 223, 224, and 225. The directions of edges 221-225 are as shown, and the inner sides of edges 221-225 are on the left.
FIG. 2b shows a printed image 200b that represents the printed image produced by a photolithography step using unmodified feature 200a in a photomask pattern. Printed image 200b shows undesirable corner rounding and shortening in region 230. To compensate for such distortion, various OPC modifications can be incorporated into the photomask layout, depending on the specific dimensions of feature 200a and its relationship to the rest of the IC layout. FIG. 2c shows a modified feature 200c, which includes a hammerhead 240 to correct for the distortions shown in FIG. 2b. FIG. 2d shows an alternative modified feature 200d, which includes serifs 250 as a corrective mechanism.
Determination of the appropriate OPC correction is made through the use of a bias table. Created prior to performing the OPC operation, the bias table defines the rules to be applied, each of which is a function of the edge properties. The bias table is typically structured as a look-up table, such as shown in Table 1.
Table 1 includes sample values for rules that would be applied to edge 223 of FIG. 2a to create OPC features 240 and 250, shown in FIGS. 2c and 2d, respectively. In Table 1, xe2x80x9cLxe2x80x9d represents the length of edge 223, and d represents the minimum spacing between edges (i.e., the distance between edge 223 and edges 221 and 225). As indicated by the values in Table 1, if feature 200a is tall and thin (i.e., less than 0.25 xcexcm wide and greater than 1.5 xcexcm tall), then a hammerhead feature will be applied. However, if feature 200a is more squat (i.e., 0.5 xcexcm wide and 1.0 xcexcm tall), serifs will be added. Of course, the bias table would contain additional sizing and placement information for the varios biases.
Because a simple lookup table can be used, rule-based OPC is much more computationally efficient than model-based OPC. However, the current edge-based systems may be too limited to effectively provide corrections for complex layouts. For example, it may be desirable to apply (or not apply) different OPC modifications to edges that have similar properties. FIG. 2e shows a feature 200e that includes an edge 263. Edge 263 may have the same length and spacing as edge 223 in FIG. 2a, but it may be desirable to apply OPC correction to only one of the two features. For example, feature 200a may be a non-critical feature that can tolerate substantial distortion, while feature 200e must be accurately transferred for proper IC function. If feature 200a was prevalent throughout the IC layout, it would be preferable to avoid complicating the final photomask layout by adding unnecessary OPC modifications. However, an edge-based system using a single edge is not able to readily make a distinction between edges 223 and 263, and therefore would not be able to apply different OPC modifications to features 200a and 200e. 
Accordingly, it would be desirable to provide a system that enables greater flexibility and control in applying layout modifications.
Embodiments of the invention provide a system for applying layout processing to an IC layout (a portion of the layout or the entire layout) using a shape-based identification system. A shape can be defined as a set of associated edges. Therefore, a shape can provide much greater specificity than a single edge in identifying layout features of interest. A catalog of shapes can be defined and layout processing actions can be formulated based on the properties of the various shapes. Shapes can include various contiguous edge profiles, such as fingers, hammerheads, diamond hammerheads, fuzzy hammerheads, and tombstones, among others. Shapes can also include non-contiguous edge combinations, edges from multiple layers of an IC layout, and edges with specific properties. Properties inherent in a particular edge can include length, inner color, and outer color, among others. Other properties associated with an edge can include spacing, beginning angle, and ending angle, among others.
While length and spacing are relatively self-explanatory, other edge properties such as inner/outer color and beginning/ending angle cannot be easily defined without understanding the concept of direction. Direction is a convention used to define the inner and outer sides of an edge. Because all edges are part of a closed form, one side of the edge will always be within the form (inner side), while the other side will always be outside the form (outer side). Therefore, direction can be defined by stating that when travelling from the beginning to the end of the edge, the inner side will always be to the left. Of course, the inner side could just as well be defined to be always to the right. The closed nature of the forms also means that each edge is joined to another edge at its beginning and its end. These junctions are defined as vertices, or corners. Each vertex represents a meeting of two edges at a specific angle. The beginning angle is the angle between the inner sides of the edges meeting at the vertex at the beginning of the edge. Similarly, the ending angle is the angle between the inner sides of the edges meeting at the vertex at the end of the edge. Finally, color is simply another convention used to identify the nature of the shape on the inner or outer side of the edge. Colors can be specified for various device or interconnect components, to enable more appropriate OPC rule application (e.g., edges forming a gate may be blue, while edges forming a diffusion region may be red).
The library of layout processing actions associated with the shapes can be rule-based, model-based, or can provide any other response a user would like implemented (i.e., xe2x80x9clayout processingxe2x80x9d can include OPC, phase shift mask (PSM), design rule checking (DRC), xe2x80x9cfracturingxe2x80x9d of layout features for e-beam mask making machines, etc.). Specific features can be much more simply, efficiently, and flexibly identified by a single multi-edge structure than by a single edge with multiple relational variables. Therefore, a shape-based feature identification system advantageously enables efficient application of layout processing actions having a high degree of specificity.
In some embodiments of the invention, the library of layout processing actions can be compiled in a bias table in an accessible format, such as a look up table. The IC layout is scanned, and whenever a shape match is detected, the associated action(s) are applied to the matching portion of the IC layout to form the corrected photomask layout. This action application process can be performed serially (i.e., after each match, the corresponding action is performed) in batch mode (i.e., all shapes are matched, after which all actions are applied), or in any desired combination of the two.
The shape matching, or xe2x80x9cscanningxe2x80x9d, operation can be performed in any number of sequences. In an embodiment of the invention, the scanning operation can be performed in order of decreasing shape complexity, where no feature in the IC layout can be modified more than once. In another embodiment of the invention, scanning can be performed in order of increasing complexity, where the most complex shape matching/layout modification is performed last. In an alternative embodiment of the invention, the order of scanning can be set by the user as desired.
According to an aspect of the invention, resolution logic may be provided to resolve shape and action conflicts. In an embodiment of the invention, the resolution logic comprises a first-shape/action-controls methodology, which can be implemented through an edge-marking technique. According to another aspect of the invention, the resolution logic comprises a set of resolution rules that mandate a desired outcome when any conflict arises.
According to an aspect of the invention, the catalog of shapes and the library of layout processing actions may be provided by the system as a default. In another embodiment of the invention, the library of shape-based rules can be modified in part or created in whole by the user. According to another aspect of the invention, the shapes and actions may be received from a remote source such as a remote server.
In an embodiment of the invention, the bias table can also include single-edge xe2x80x9cfiller shapesxe2x80x9d that provide default layout modification for portions of the IC layout not covered by the multi-edge shapes. In another embodiment of the invention, the bias table can include both rule-based OPC actions and model-based OPC actions.
The invention will be more fully understood in view of the following description and drawings.